Simple Li-Fi Transmitter Receiver Circuit

Think of the internet as light instead of radio waves.

Exactly that is what Li-Fi technology achieves.

Building a simple Li-Fi system to transmit and receive data using light is explained in this article.

A bright LED and a standard transistor are used by the transmitter to convey the signal.

The receiver picks up the signal using a specific light sensor such as a solar cell and enhances it so that it may be heard clearly from a speaker.

This is an easy project to learn how Li-Fi operates but it wont take the place of your Wi-Fi.

What is a Li-Fi Transmitter Receiver Circuit:

Light Fidelity or Li-Fi is a wireless communication method that transmits data using infrared or visible light.

Li-Fi systems use light sources usually LEDs to transfer data that has been modulated into light waves.

The sent data is obtained by a Li-Fi receiver that captures and processes the modulated light signals.

Transmitter Working:

Parts List:

The 2N2222 transistor is configured as a common emitter amplifier.

The voltage divider at the base VBias is developed using resistors R1 and R2​.

Audio input to the transmitter causes variations in the transistors output.

These variations modulate the brightness of the 1 watt LED in response to the audio signal.

The audio signals quality is maintained by capacitors at each transistors base that stop DC signals from passing through.

If operating at higher voltages e.g.12V a current limiting resistor in series with the LED ensures proper working.

Formulas and calculation for transmitter working:

The formula below calculates the base voltage VBias in a voltage divider biasing circuit for a transistor:

V_Bias = R₂ / (R₁ + R₂) × V_Supply

here,

• V_Bias represents the voltage at the base of the transistor which is the voltage we are trying to calculate.
• R₁ & R₂ are the resistances of the two resistors used in the voltage divider circuit.
• V_Supply is the supply voltage applied to the entire circuit.

Using the voltage divider concept the formula correctly calculates the voltage across resistor R2 which is connected to the transistors base.

The closer V_Bias is to VS_upply the larger the ratio of R2 to (R1 + R2).

Steps to Calculate VBias:

Substitute the given values:

• R1 is 4.7kΩ (kilo-ohms)
• R2 is 1kΩ (kilo-ohms)
• VSupply is 9V

Calculate the total resistance:

Rtotal = R1 + R2 Rtotal = 4.7kΩ + 1 kΩ Rtotal = 5.7kΩ

Calculate the voltage divider ratio:

Voltage divider ratio = R2 / Rtotal

Voltage divider ratio = 1kΩ / 5.7kΩ

Voltage divider ratio = 0.175

Calculate VBias:

VBias = Voltage divider ratio × VSupply

VBias = 0.175 × 9V

VBias = 1.57V

Therefore in this example the base voltage (VBias) is approximately 1.57V

Note:

This formula assumes ideal resistors, perfect conductors and no voltage loss between them.

Small variations may arise in actual situations due to resistor tolerances.

The voltage divider biasing method is an easy and efficient way to change a transistors base voltage.

The resistance values used will decide the transistors operating point in the circuit.

Transmitter Construction:

To construct the transmitter use a 2N2222 transistor configured as a common emitter amplifier.

This alteration in LED brightness corresponds to the audio signal received.

Include capacitors at each transistor base to block DC signals and preserve audio signal quality.

For operation at higher voltages include a current limiting resistor in series with the LED.

Adding a audio sources include mp3 players, mobile phones or microphones with pre amplifiers.

Receiver Working:

Parts List:

The receiver offers a solar cell in series with a 2.2μF capacitor.

The solar cells voltage output is sensitive to variations in ambient light.

The varying voltage from the solar cell is provided into an amplifier LM386 or similar.

The amplifiers sensitivity ensures it can pick up the relatively weak audio signal.

The amplified audio signal is then sent to a speaker producing clear sound.

Receiver Construction:

The receiver involves a 6V solar cell (3V will also do) in series with a 2.2μF capacitor combined with an amplifier.

While the amplifier can vary opt for one with excellent sensitivity.

Examine the circuit in a room with low lighting and no electrical lights close by.

Formulas:

An audio amplifier using a single transistor and a transformer are mentioned in below formulas:

The transformers impedance ratio is the square of the turns ratio (Np / Ns):

Impedance Ratio = (N_p / N_s)²

here,

• N_p is the number of turns in the primary winding
• N_s is the number of turns in the secondary winding

Note:

A high quality audio amplifier needs to be carefully designed and assembled using the right parts.

Use multi stage amplifier circuits with right biasing and filtering methods for best performance.

Overall Circuit Operation:

The circuit can function in low light conditions with little interference from electrical light sources because of its design.

The LED might flicker a little but it wont be visible to the human eye.

Although these changes are not visible to the human eye the brightness of the LEDs changes somewhat when audio is sent into the transmitter.

These slight voltage variations are captured by the receivers solar cell which enables the amplifier to generate unique and audible sound.

The circuit can be tested by making sure the transmitter and receiver are powered on giving the transmitter audio input and adjusting the volume.

The speaker on the receiver should produce clear audio.

Building and running a basic Li-Fi circuit for wireless audio transmission is possible if you understand and put these ideas into action.

Procedure:

Assemble the transmitter and receiver circuits separately.

Connect the 1 watt LED in parallel with the solar cell.

Power both transmitter and receiver and adjust the transmitters volume.

Provide audio input to the transmitter.

In a low light environment listen for clear audio from the receivers speaker.

Experiment with a photodiode in the Li-Fi circuit replacing the amplifier section with an LM386 amplifier circuit.

The circuit continues to function well providing option in component choices.

Important Notes and Considerations:

There may be slight flashing in the LE, which could be a sign of possible problems.

By using audio input small changes in LED brightness that are invisible to the human eye can take place.

The LED stays solid ON when there is no audio input because DC signals are blocked by the input capacitor.

By mimicking tiny voltage changes the solar cell rejects steady DC voltage and allows the capacitor to transmit the audio signal.

Choose a sensitive amplifier to improve the circuits performance.

Conclusion:

You may create a dependable Li-Fi circuit for wireless audio transmission by following these steps providing a different method of communication in areas with poor lighting.

For maximum performance based on what you require try out various components.

References:

System architecture of Li-Fi transmitter and receiver.

Li-Fi